1. Field of the Invention
The present invention relates to image processing, and more particularly, to a method and apparatus for correcting chromatic aberration of an image, the method and apparatus used in an image processing apparatus for an image pickup device or a display image processing apparatus.
2. Description of the Related Art
Generally, when an image is photographed using a camera, there occurs various problems due to aberration of a lens. In particular, image deterioration called chromatic aberration is a serious problem. Chromatic aberration is a phenomenon in which false colors are generated since different wavelengths of light are focused at different positions due to a lens having a different refractive index for the different wavelengths of light. When an image is photographed using a high-magnification lens or an inexpensive lens, such chromatic aberration becomes more apparent. Chromatic aberration may be caused simply by a defect of a lens. However, chromatic aberration may also be caused by a combination of defects of an image sensor, such as a charge coupled device (CCD), and a lens.
In a region near edges of an image, significant differences in luminance appear suddenly. In this edge region, a defect of an image sensor, called blooming, as well as a defect of a lens occurs. Generally, blooming causes chromatic aberration which has very high chrominance (e.g., purple fringes) deflected by a lens. Various types of chromatic aberration may also be caused by a combination of characteristics of a lens and an image sensor.
After going through an image processing process, which is referred to as image restoration and used in digital auto focus technology, an edge region may have more serious chromatic aberration or color artifacts. This is because a degree of image restoration varies according to each wavelength. That is, since edges of an image do not have a degree of blur after an image restoration process, chromatic aberration, which has already occurred, becomes more noticeable.
In order to reduce such chromatic aberration, various methods, such as using a lens made of a special glass material or using a specially processed lens, have been suggested. However, when these methods are used, the cost of manufacturing or processing a lens is increased. Thus, it is difficult to widely use the above methods in cameras other than high-performance cameras such as digital single-lens reflex (DSLR) cameras.
In this regard, it may be more efficient to use technology for removing or correcting chromatic aberration, which is generated in an image already photographed, through image processing. Conventional methods of removing or correcting chromatic aberration through image processing are disclosed in Japanese Patent Publication Nos. 2006-115039 and 2000-299874.
Specifically, Japanese Patent Publication No. 2006-115039 discloses technology for calculating a false color degree, which is defined as a parameter indicative of the possibility that a central pixel of an input image is represented in a false color, and calculating an interpolation value of the central pixel based on values of pixels in a region near the central pixel. The false color degree determines a weight of each pixel for processing a chrominance value, and chromatic aberration is corrected using the weight of each pixel and the false color degree. However, the technology cannot be used to correct chromatic aberration showing various colors which occurs in almost all edges.
In a method of correcting chromatic aberration disclosed in Japanese Patent Publication No. 2000-299874, a reference pixel is designated in an input image, and an amount of chromatic aberration according to the distance from a reference position of the reference pixel is detected. Then, a chromatic aberration characteristic curve representing a cubic equation of the detected amount of chromatic aberration is created, and chromatic aberration is corrected based on the curve. However, the method is designed to correct only transverse chromatic aberration among various chromatic aberrations. In addition, a separate chromatic aberration characteristic curve has to be created for the amount of chromatic aberration which varies according to the distance from the reference position.